Surface and interfacial tensions are called “static” and “quasi-static” when their values, under constant external conditions, do not change, stop changing, or change very slowly. They are therefore in or very close to thermodynamic equilibrium. To determine this, a ring (Du Noüy method) or a plate (Wilhelmy method) is placed in contact with the surface of the liquid and the resulting force is measured. This force is a measure of the surface/interfacial tension. The geometry of the measuring body is set by standards, e.g. ATM D971 and DIN 53914.
The Du Noüy method is preferred over the plate method despite greater effort (correction of measured value), as it gives triple the resolution. Another reason is simpler cleaning of the more robust measuring body, as well as the lower resolution. The ring method offers particular advantages in fully automated measurements, such as the automatic determination of concentration dependencies (CMC) and in quality control.